(1) Field of the Invention
The present invention relates to a document feeder and also to a document reader and an image forming apparatus each having the document reader.
(2) Description of the Related Art
Many image forming apparatuses, such as copiers, are equipped with a document feeder that picks up a document sheet placed on a document tray with pick-up rollers, transports the document sheet to an image reading position, and ejects the document sheet onto a document output tray.
Assuming that a stack of document sheets contain different sizes of sheets, some document feeders are configured to transport each document sheet in a so-called “one-edge positioning” manner according to which each document sheet is transported while one edge of the document sheet in a width direction is guided along a guide plate. With such document feeders, it is inevitable that the pick-up roller is pressed against a document sheet at a position deviated from the center of the width (hereinafter, the center of the width may also be referred to as “widthwise center”) when advance the document sheet.
When a document sheet is pulled by pressing the pick-up roller against the document sheet at a position deviated from the widthwise center, it often happens that a turning moment is exerted on the document sheet and thus the document sheet is often skewed.
In general, a document feeder is provided with a pair of registration rollers located downstream from the pick-up roller for skew correction. However, if the skew occurred at the time of pulling the document sheet is too large, the pair of registration rollers may not be able to completely correct the skew.
In view of the above, there is a demand for such document feeders that minimize skew occurring when the pick-up roller pulls a document sheet.
Although relating to the feeding of recording sheets from a paper feed cassette in an image forming apparatus, Japanese Patent Application No. 2009-292598 (hereinafter, “Patent Literature 1”) suggests a separator mechanism located downstream in the paper feed direction to correct skew of the recording sheet being fed.
The separator mechanism is formed from a cylindrical paper-feed roller and a cylindrical separation roller. The paper-feed roller is driven to rotate, and the cylindrical separation roller is applied with a constant load torque and pressed against the paper-feed roller. According to Patent Literature 1, the rotation axis of the paper-feed roller is tilted relative to the rotation axis of the separation roller by a predetermined angle. As a result of the tilting, the position on the paper-feed roller at which the separation roller makes contact shifts along the direction of the rotation axis, so that skew correction is made.
In the manner described above, by applying the structure of tilting the rotation axis of the roller to the pick-up roller of the document feeder, the position on a document sheet at which the pick-up roller makes contact is ensured to be closer to the widthwise center of the document sheet, which is considered effective to suppress occurrence of skew.
Unfortunately, however, the following should be noted with respect to the image forming apparatus described in Patent Literature 1. That is, when the paper-feed roller and the separation roller are axially in parallel, the respective rollers are pressed against each other uniformly across the width, which means that the nip pressure is maintained at a predetermined level. However, when the paper-feed roller is tilted, only a limited portion of the paper-feed roller near the tilted edge makes contact with the circumferential surface of the separation roller, which means that the nip pressure increases locally and significantly.
For this reason, if the rotation shaft of the pick-up roller is made to tilt by simply applying the structure of Patent Literature 1 to the pick-up roller, the following problem arises. That is, the pressure at a portion of a document sheet pressed by the pick-up roller increases locally, which leads to an increase in friction between the uppermost document sheet and the document sheet below the uppermost document sheet. In this manner, a substantial transport force is applied also to one or more document sheets below the uppermost document sheet, which results in that multiple document sheets are fed at a time (multi-feed).
Generally, a pair of separation rollers is provided at a location downstream from the pick-up roller. Yet, the separation ability of the rollers is limited. When too many sheets are fed at a time, the separation rollers alone cannot reliably separate a single document sheet from the rest.